The invention relates to a compressor for an exhaust gas turbocharger.
The operating range of compressors in exhaust gas turbochargers is defined by a surge limit and a choke limit which are identified by limit lines in a performance graph characterizing the compressor behavior. During operation of the compressor near the surge limit, locally limited stall zones may occur at the blades of the compressor wheel, which may result in pulsating of the flow with a periodic alternation of the flow direction. The surge limit of the compressor is reached when ever smaller volume flows are delivered through the compressor.
When, on the other hand, the operating point of the compressor is shifted to progressively higher volume flows in the compressor performance graph, the choke limit may be reached, which is characterized in that the flow volume at the wheel inlet is blocking the entrance to the compressor wheel.
For expanding the operating range of the compressor, performance stabilizing measures are known which shift both the surge limit and the choke limit so as to provide for an expanded operating range.
For example, U.S. Pat. No. 4,212,585 A shows a performance graph stabilizing measure according to which slots are formed in a casing of the compressor around a circumference of a compressor wheel inlet. The slots have a certain length, with a first end of the slots being positioned upstream of a compressor wheel leading edge and a second end of the slot being positioned downstream of the compressor wheel leading edge. In other words, the slots extend axially beyond the compressor wheel inlet edge.
European Patent Specification EP 0 614 014 B1 also discloses an arrangement of slots in the compressor casing, which slots are provided around the inlet of the compressor wheel. The slots are positioned upstream of a leading edge of the compressor wheel and are formed with a depth almost in parallel to the compressor wheel circumferential edge to improve their effect. This means, it is intended to obtain a slot beginning which is arranged slightly upstream of the compressor wheel circumferential edge.
It is the object of the present invention to provide an improved compressor for an exhaust gas turbocharger.